RF filters are employed in RF circuits and devices to filter particular bands of frequencies and/or to remove unwanted or spurious noise. As greater demands are placed on RF systems, for example in WLAN's, in order to increase channel capacity by utilizing available bandwidth, corresponding demands are placed upon performance and tolerance of components used in the RF circuits.
HIPERLAN 2 is an example of a standard that has been developed in order to increase channel capacity. HIPERLAN 2 is a Wireless Local Area Network (WLAN) protocol, based on, and incorporating an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
An advantage of using an OFDM scheme for transmitting RF signals is that it is possible to transmit more data for a given channel bandwidth than was previously possible. The following modulation formats may be incorporated in the HIPERLAN 2 scheme: Binary Phase Shift Keying (BPSK); Quadrature Phase Shift Keying (QPSK); Quadrature Amplitude Modulation (16 QAM) and 64 QAM. The flexibility of use of the aforementioned modulation techniques means that data rates of between 6 to 54 Mbit/s can be transmitted.
A particular advantage of the HIPERLAN 2 scheme, is that systems incorporating it may be used in offices, shops, airports or in similar environments, which were previously prone to multipath dispersion. This is because RF transmissions which use HIPERLAN 2 are reflection resistant.
Typically HIPERLAN 2 operates at 5.5 GHz with multiple channels; each channel has 52 active sub-carriers within a 20 MHz bandwidth. Eight HIPERLAN 2 channels are shown in diagrammatical form in FIG. 3. OFDM modulation schemes require two component signals to be in phase quadrature. The two component signals are referred to as the I and Q signals. During operation, and following down conversion to a base-band signal, the base-band signal of interest has two 10 MHz side bands, at which sub-carrier frequencies, the I and Q component signals lie.
However, because only a relatively small frequency band separates adjacent sub-carrier channels, even small changes in filter characteristics can cause variations in overall filter behaviour, with the result that small variations in filter characteristics (for example as a result of thermal drift) give rise to cross-talk interference between adjacent sub-carriers. Typically a change in overall absolute tolerance of ±16% of the passive components' value prevents the filter from achieving the required of −27 dB stop band performance. FIG. 3 shows how sub-carriers are separated from a channel. FIG. 4 illustrates diagrammatically how close together the sub-carrier channels are one to another.
It would therefore be advantageous to be able to adjust the roll-off characteristics of two analog low-pass filters so as to meet the performance requirements specified in HIPERLAN 2.